The present invention relates to nondestructive eddy current testing of metallic workpieces, and more particularly, to a method and apparatus for filtering from the defect signal any reoccuring noise present therein.
In the art of nondestructive testing, the use of eddy current techniques and equipment is well known. More specifically, eddy current means have been used to detect various defects in tubular members, such as cracks, inclusions, thinning and the like. In the typical nondestructive test of tubular member using eddy current techniques, an eddy current probe is passed through the tubular member to detect any defects in the wall of the tubular member. The eddy current probe includes sensing means for detecting the defect and for generating a defect signal representative of the defect and informative of the various characteristics of the defect such as depth, size and location.
One particular application of nondestructive eddy current testing is the inspection of the typically long and coiled heat exchange tubes of steam generators and process heaters. One problem frequently encountered when inspecting such heat exchange tubes is the presence of noise in the defect signal from the eddy current probe sensing means which can frequently mask smaller defects which, if undetected can lead to potential catastrophic failures at a later date. Much of the noise is generated from anomalies in the inner surface of the tube resulting from manufacturing operations such as plug chatter, reeling marks, pilgering and the like. Additionally, if the heat exchange tubes are finned on their outer surface, a noise signal is generated by the presence of the fins. In order to avoid problems with repetitive noise in the tubes, manufacturers have gone through elaborate manufacturing operations to ensure such anomalies are not present in their product and have been forced to eliminate fins on applications where inspection is necessary.
One known technique for handling the noise generated by such tube anomalies is termed multicurrent or multifrequency eddy current testing. In this known technique, a second current or frequency is utilized to generate a second defect signal having a different frequency than the first defect signal. The two signals are then processed in a manner such as disclosed in U.S. Pat. No. 4,061,968 through elimination circuits to remove the undesirable noise.
However, such multifrequency, eddy current testing is by its very nature more difficult, time consuming and expensive than single frequency eddy current testing. Therefore, it is an object of this invention to provide a single frequency eddy current apparatus which is capable of filtering any reoccurring noise out of the defect signal.